Disk-shaped splats that can be obtained on a heated substrate reveal a better contact with the substrate surface, resulting in a better adhesion/cohesion of coatings. Some research results illustrate that when the substrate temperature exceeds the transition temperature, the ideal splat can be obtained. In the simultaneous preheating-spraying-cooling process, named HEATCOOL in which a preheating gun heats the specimen just before the spraying and a cooling jet cools it just after the spraying, the preheating temperature is of short duration. In order to study the preheating effect and determine the optimal velocity of the movement of the system for obtaining an appropriate preheating temperature, the function specification method, an optimisation method, was adopted combined with FEM (the finite element method). An oxy-acetylene flame was used as preheating thermal resource, the optimal velocities were estimated. Particle impact tests of Cu and YSZ (ZrO2- 8%Y2O3) powders sprayed by plasma with a flame heating system were carried out with different moving velocities. The splats morphology collected on stainless steel and aluminium plates were observed to clarify and confirm the calculated results.